<!DOCTYPE html>
<html lang="zh-CN;">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>画板</title>
  <link rel="stylesheet" href="/style.css">
</head>
<body>
  <canvas id="canvas" width="100" height="100" ></canvas>
  <script>
    let canvas = document.getElementById('canvas')
    canvas.width = document.documentElement.clientWidth
    canvas.height = document.documentElement.clientHeight
    //画线
    let ctx = canvas.getContext('2d')
    let painting = false
    let last
    ctx.fillStyle = 'black'
    ctx.strokeStyle = 'none'
    ctx.lineWidth = 5
    ctx.lineCap = 'round'

    function drawLine(x1, y1, x2, y2) {
      ctx.beginPath();
      ctx.moveTo(x1, y1);
      ctx.lineTo(x2, y2);
      ctx.stroke()
    }


    var isTouchDevice = 'ontouchstart' in document.documentElement
    if(isTouchDevice){
      canvas.ontouchstart = (e) => {
        let x = e.touches[0].clientX
        let y = e.touches[0].clientY
        last = [x, y]
      }
      canvas.ontouchmove = (e) => {
        let x = e.touches[0].clientX
        let y = e.touches[0].clientY
        drawLine(last[0], last[1], x, y)
        last = [x, y]
      }
    }else{

      canvas.onmousedown = (e) => {
        painting = true
        last = [e.clientX, e.clientY] 
      }

      canvas.onmousemove = (e) => {
        if (painting === true){
          drawLine(last[0], last[1], e.clientX, e.clientY)
          last = [e.clientX, e.clientY]
        }
      }
      canvas.onmouseup = () =>{
        painting =false
      } 
    }
  </script>
</body>
</html>